One adjustment which still generally must be made in most television receivers is a fine tuning adjustment. Frequency synthesizer tuning systems having a wide pull-in range and operating to automatically correct for frequency offsets without affecting the operation of the conventional frequency synthesizer in the tuning system have been developed. In addition, systems exist which permit manual fine tuning adjustments to override the frequency synthesizer tuning adjustments when such overriding manual tuning is desired.
These systems represent a substantial improvement over systems of the prior art which either used solely a manual fine tuning adjustment or which merely used a conventional frequency synthesizer system without a frequency offset correction provision. In the past, there has been some interest in providing a television receiver with a "signal seek" feature, that is, a tuning system which, upon command, seeks the next available transmission signal, skipping intermediate unused channels or channels which do not have a signal broadcast on them at the time the receiver is being operated. A signal seek tuning system which is compatible with a frequency synthesizer tuning system having an automatic offset correction feature is disclosed in the above-identified related patent.
In the frequency synthesizer tuning system with signal seek disclosed in U.S. Pat. No. 4,041,535, a compromise must be made between the signal seek search speed and accurate tuning acquisition. As the search speed is increased, tuning acquisition accuracy deteriorates, particularly for difficult channel transitions such as channel transitions fron one band to another (low VHF band to high VHF band or UHF, high VHF band to low VHF band or UHF, and vice-versa).
The transitions for changes between channels in different tuning bands or between channels located at extreme ends of the same tuning band create ringing in the tuning voltage; so that it is possible to obtain a momentary channel verification and then lose it one or more times before the system settles down. If the decision for channel acquisition is made at the time verification has been momentarily lost, the signal seek system would advance to the next channel location even if there was a station present. In the past, this problem has been dealt with by simply slowing down the signal seek operation across the entire spectrum of channels to allow sufficient time for interrogating every channel (thus lengthening the dwell time at each step-by-step interrogation to the length required for the worst case). Since for most channel-to-channel stepping, however, the problem does not exist, a faster search speed (a shorter interrogation or dwell time per channel) permits reliable acquisition; and a faster search speed is desirable to permit the system to rapidly skip over channel locations where no signal is present.
It is desirable to incorporate into a frequency synthesizer tuning system having a signal seek function, a provision for selectively lengthening a normally short interrogation or dwell time whenever the channel-to-channel stepping from a previous channel to a new one is one which most likely requires a longer dwell time; but to do this only for such stepping intervals which are likely to have this problem.